Apache 2.4.9 – status 255, faulting mod ule: ntdll.dll

Hi! I run a local server using WAMP (Apache Version 2.4.9 Win32, PHP Version 5.5.12, MySQL version 5.6.17) on Windows 7 (with administrator rights). Sporadically (maybe once every three hours, but very unpredictable), the connection to one of the client dies with the following message in the apache error log (status 255): [mpm_winnt:notice] [pid 668:tid 328] AH00354: Child: Starting 64 worker threads. [mpm_winnt:notice] [pid 728:tid 412] H00428: Parent: child process 668 exited with status 255 -- Restarting. [mpm_winnt:notice] [pid 728:tid 412] AH00455: Apache/2.4.9 (Win32) PHP/5.5.12 configured -- resuming normal operations [mpm_winnt:notice] [pid 728:tid 412] AH00456: Apache Lounge VC11 Server built: Mar 16 2014 12:13:13 There are no error messages in the php error log. In the windows event log I get the following messages: Faulting application name: httpd.exe, version: 2.4.9.0 Faulting module name: ntdll.dll, version: 6.1.7601.18798, time stamp: 0x5507b3e0 Exception code: 0xc0000005 Fault offset: 0x0002e3be Faulting process id: 0x29c Faulting application start time: 0x01d16af6d52655e1 Faulting application path: C:\wamp\bin\apache\apache2.4.9\bin\httpd.exe Faulting module path: C:\Windows\SysWOW64\ntdll.dll And further: Event Name: APPCRASH Response: Not available Cab Id: 0 Problem signature: P1: httpd.exe P2: 2.4.9.0 P3: 532585f0 P4: ntdll.dll P5: 6.1.7601.18798 P6: 5507b3e0 P7: c0000005 P8: 0002e3be P9: P10: When this happens the client becomes unresponsive but immediately starts to work again after some seconds. In total I have 24 parallel clients that interact with the server. Again, it only happens sporadically to single clients. I found some old threads related to this issue, but often related to other "Faulting Modules". Can someone help me on this issue? I am completely lost with regards to whether this is related to a (i) php issue, (ii) windows issue or (iii) apache issue and how to attempt to solve it. all the best!